July 1924 saw the deep lava recovering in the crack and sealing off the water, so as to bubble up in the bottom debris of Halemaumau and push its way upward into the crevices of the island. It reached the top of Mauna Loa in 1926 and reactuated outflow at the center of the island. This migration of vents from top to bottom and back again took twelve years of fracturing, and it relieved from lava this big piece of the Hawaiian ridge. In reaching the groundwater and steamblast phase, it accomplished something which had not happened since 1790, making a supercycle of 134 years.
The decade after explosion at Halemaumau was marked by small lava gushes in the bottom of the pit, bringing the depth from 1,300 feet in 1924 to 750 feet in 1934. The layers were something less than 100 feet each, and they were fed by pahoehoe conelets at the slide-rock margin. As usual, the lava was gushing up the western wall crack along the margin of the bottom magma cylinder. There was no trace of recurrence of steam blasts.
Despite the excitement of actual events, experimentation continued; and I continued working on inventions for the experiments. Two approaches to our problems concerned seismic recorders which could be put in the hands of amateurs, and range finders for improving pit surveys. I had been convinced for many years that the three-component seismograph was too elaborate to be operated by volunteer school teachers or telephone operators who have other things to do. Such a seismograph records with photographic paper the north-south, east-west, and up-down motion of the ground, on a chronograph which keeps accurate time and registers a wavy line every second, so that the recording paper has to be changed and developed every day. Moreover, these instruments are for measuring distance to earthquake origins by physics of wave motion, and they have become hopelessly mathematical. Such mathematics makes for assumptions of uniformity about a rock crust which is not uniform. Qualitative science wants to know what happens at a specific rock location and wants the motion recorded by the simplest possible mechanical device. It also wants a value in number at each location, for size and direction of the first motion. This is for an earthquake, identified as one incident, over such an island as Hawaii, where the rock units are many and different. This is especially true of long periods of time when there may be no earthquakes to record.
I devised a simple shock recorder, consisting of a horizontal boom of very light wood attached to a hinged weight which swung like a door, so that the boom scratched a line on a circular card which was rotated and moved along by a common alarm clock. The result was a spiral mark on the card, such that an earthquake interposed would write a zigzag opposite a place on the clock face appropriate to the time of day. All that was necessary was to remove and date the card, wind the clock once a day, and measure the zigzag.
Mr. Ingalls of Scientific American read an article by me in which I described my shock recorder and thought it would lead amateur machinists to devise their own machines and to record the vibrations about them. Numerous amateurs did send in designs for instruments, and Ingalls believed that the seismograph hobby would become as popular as the amateur astronomical telescope hobby. But it failed because the amateurs were waiting for earthquakes, which didn’t happen. They were not content with vibrations from trucks, railroad trains, waterfalls, surf on rocks, artillery practice, or wind storms.
My improved shock recorder gained some use later in New Zealand and Montserrat, after big earthquakes in those places stirred the authorities to build simple instruments. However, popular seismoscope simply doesn’t exist.
The range finder I had been working on since my teaching days in Massachusetts Tech, where I had made an optical device with a traveling index mirror which moved along an upright scale of centimeters, and a sextant telescope. The idea was a transit, with self-contained base line close to the operator. My theory was that in such measurements of distance as we had to use—to about a thousand feet or less, to the lava fountains in the bottom of Halemaumau pit—we might read off the vertical distance from a single station, when all other stations were enclosed in smoke.
In the Aleutian Islands and elsewhere I experimented with a Zeiss stereoscopic rangefinder designed for artillery ranges, but it was not accurate enough for short distances. Everything in my instrument depended on moving a telescope parallel to itself with superlative precision, on a scale within the instrument. I finally hit upon using a track of taut piano wire, probably the straightest line in all mechanics.
If one first looked at an object twenty miles away (infinite distance), the telescope could be moved along right and left and the image would remain immovable on a vertical hair. If it were now focussed on an object 1,000 feet away, the displacement of the telescope on the centimeter scale would measure the distance with a high degree of accuracy. This was the stadia principle inverted to contain the rod at the observing position.
I also made several graphic devices for surveying Halemaumau daily from the rim benchmarks. However, when lava overtopped the rim and destroyed the datum posts, mapping became difficult.